A fantastic new paper published recently in Current Biology (Cinner et al. 2009) examines the correlations between a variety of human demographic, social, cultural and economic indicators and reef fish biomass.  The somewhat surprising and novel result is that fish depletion is maximized at intermediate levels of human socioeconomic development.  The authors, mostly based at James Cook University’s Centre of Excellence for Coral Reef Studies, surveyed 19 coastal communities adjacent to coral reefs in the Indian Ocean. 

In fished sites, fish biomass was negatively related to human population density, but it was best explained by reef complexity and a U-shaped relationship with socioeconomic development. The biomass of reef fishes was four times lower at locations with intermediate levels of economic development than at locations with both low and high development.

An important underlying mechanisms was the changing availability of technologies as communities become wealthier:

…in low-development sites, technological constraints and social institutions may limit people’s exploitation of marine resources. Reduced dependence on marine resources, variable access to boats but increasing access to engines and other technologies, high use of spear guns, and a lack of customary management institutions characterize communities with intermediate levels of development. 

One thing that didn’t surprise me was that human population density was only weakly related to fish biomass and wasn’t even included in the best fit model:

A key and surprising finding from this study is that the best model included the quadratic socioeconomic-development index and reef structural complexity, but did not include human population density. 

There is a pretty big debate among coral reef and other marine scientists about whether human population size (and growth) per se is the ultimate problem (as my buddy The Natural Patriot believes) or whether the impacts of societies have a lot more to do with governance and policy, local choices and customs, and the application of technologies.  As I have written about on ClimateShifts in the past, I think simply blaming the problem on human population growth is silly, i.e., condoms are not going to  save coral reefs.  Stephen Jameson published a provocative essay addressing this debate and I still think there is plenty of evidence to suggest that we cannot mitigate most threats to the ocean simply by limiting or reducing the number of people on earth.  In the context of the Cinner et al paper, where would a human population cap put us along the development index?

One of the other things that struck me about the results was the relative weakness of the effect of the human indicators on fish biomass compared to unfished reefs in fully protected reserves (Fig 3): across a range of human population density and socio-economic indicators, people flat out totally deplete coral reef fish. I wonder what this means for the purported links between the development index, macroalgae, coral recruitment and reef resilience laid out by Bob Steneck in his companion piece (Steneck 2009; see the figure from Steneck’s article about the Cinner et al. paper below).  Given the relatively small variance in fish biomass (only about 25% of the natural range) could there really be a predictable indirect effect of human development on coral populations? Maybe. I wonder if Cinner et al. also measured macroalgae and coral recruitment in their surveys.  

I also wonder  if  the net impact of people living in societies with a high development index (>1) is just being displaced across a region or even globally via protein imports, i.e., does the net impact continue to increase even as the local impact decreases?  Think of highly developed countries like the US and Japan that import a large portion of their animal protein. Such societies do have far higher protein consumption rates, so it seems plausible that they are simply eating fish from other peoples reefs!  It would be interesting to examine fish imports, exports and consumption at a local societal scale across such a development gradient.  

Another issue is how the other impacts to coral reefs, in particular fossil fuel consumption and global warming, are related to the development index.  They must certainly be related, but are they linearly related?  I bet it could even be a positive exponential function.  In other words, how is the development index related to other taxa and indicators of reef health or state.  For example, what about invertebrates?  Might prey populations of these harvested fish species show the opposite relationship with socio-economic indicators? And what about corals?  The enormous complication is that the impacts of many human activities (e.g., driving a car, using an electric clothes dryer, flying on an airplane to go on vacation, etc.) are regional to global and thus cannot be analytically related to local human socioeconomic parameters (i.e., the sample size of the response variable is essentially 1).   

Like most transformative research, the Cinner et al. paper raises at least as many questions as it answers. I think it is sometimes hard to gague the future impact of resarch when it first comes out-it can be difficult to impossible to envision all the twists and turns a research program will take.  But I put my money on this being a citation classic.  It has already really influenced my thinking and future research plans!  

Literature Cited

Cinner, Joshua E, Timothy R McClanahan, Tim M Daw, Nicholas A.J Graham, Joseph Maina, Shaun K Wilson, Terence P Hughes (2009) Linking Social and Ecological Systems to Sustain Coral Reef Fisheries.  Current Biology 19:206-21

Jameson SC (2008) Guest editorial: Reefs in trouble  the real root cause. Marine Pollution Bulletin 56(9):1513-1514

Steneck, RS (2009) Marine Conservation: Moving Beyond. Malthus. Current Biology R117 DOI: 10.1016/j.cub.2008.12.009

Several of our San Diego-based colleagues have been making the argument that fishing is the ultimate cause of coral bleaching, disease and loss in a series of stimulating papers.  One purported underlying mechanism is that by reducing the density of herbivores, fishing is causing increases in macroalgae and concentrations of Dissolved Organic Carbon (DOC), thereby fueling microbes that lead to coral diseases or make corals more susceptible to  warming.

Evidence supporting this idea stems from somewhat artificial laboratory studies and a field survey of four remote reefs in the central Pacific (see my post on this work here).  After reading these papers, three UNC undergrads working in my lab asked “why doesn’t someone do a field experiment to test this idea?”  So they did.  Working with Ernesto Weil and I in Puerto Rico they performed three field manipulations to test the general hypothesis that overfishing and the subsequent alteration of coral reef trophic dynamics are a cause of coral epizootics.  Specifically, they asked whether the presence of macroalgae can influence within- and among-colony spread rates of Caribbean Yellow Band Disease in Montastraea faveolata.

They placed macroalgae in small pouches next to infected and healthy, adult and small coral colonies to measure effects on disease spread rate, coral growth and coral survival.  Surprisingly, the addition of macroalgae did not affect disease severity or coral fitness.  Their results (published yesterday in PLoS One; Vu et al. 2009) suggested that macroalgae have no effect on the severity and dynamics of Caribbean Yellow Band Disease, a critical coral epizootic.

There are several unresolved issues though.  First, could other algal species or other combinations of algae cause or exacerbate yellow band disease? What about other host species and other coral disease syndromes?  Could there be other unexplored factors that are also necessary for infection to occur (e.g., high temperature)?  And how is the concentration of relevant forms of DOC related to fish, fishing and algae?   All of these questions remain unanswered.  Leaving lots of room for future field experiments by ambitious students.

From CNN.com:

(CNN) — Scientists expect some great travel spots to be altered or ruined by global climate change.

Some of the changes are already taking place. Others are expected to be seen in coming decades.

There are two ways to look at this: Either stay home (which might be less depressing and won’t add more airline emissions) or get a move on it and see the hot spots you just can’t miss.

For those who want to head out, CNN got advice on the best pre-warming travel destinations from Bob Henson, author of “The Rough Guide to Climate Change” and a writer at the University Corporation for Atmospheric Research in Boulder, Colorado.

Here are Henson’s top five choices:

Great Barrier Reef, Australia

Warming temperatures can spell disaster for coral reefs, which depend on a delicate balance of ocean temperature and chemistry to bloom into colorful displays.

Many of the world’s reefs already are experiencing “bleaching” in which algae living in the coral die and leave behind whitened skeletons.

The Great Barrier Reef — which is composed of about 2,900 individual reefs and is off the northeast coast of Australia — is seeing limited bleaching now, and the Great Barrier Reef Marine Park Authority expects the problem to grow in coming decades.

See the full article here